Methods of contacting substances and microsystem contactors

ABSTRACT

A microchannel contactor and methods of contacting substances in microchannel apparatus are described. Some preferred embodiments are combined with microchannel heat exchange.

RELATED APPLICATIONS

[0001] In accordance with 35 U.S.C. sect. 119(e), this applicationclaims priority to U.S. Provisional Application No. 60/363,859 filedMar. 11, 2002.

[0002] This invention was made with Government support under ContractDE-AC0676RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

[0003] The present invention relates to processes and devices thatcontact a gas phase component with a component in a liquid phase.

BACKGROUND OF THE INVENTION

[0004] Compact systems for capturing and/or separating fluids aredesirable in a variety of applications. For example, hydrogen-poweredvehicles could utilize fuel cells that recycle water. As anotherexample, efficient and lightweight systems for recovery and reuse ofwater in spacecraft has long been recognized as a requirement for humanspace exploration. The present invention provides methods and apparatusfor efficient fluid capture and separation.

SUMMARY OF THE INVENTION

[0005] The potential applications of sorption techniques, bothabsorption and adsorption, for targeted gas separations and purificationare essentially unlimited provided sorption media with requiredspecificity and capacity are available. For example, microchannelabsorbers can be used in the recovery and recycle of CO₂ and thestripping of CO from RWGS and Sabatier reactor effluent gases. Two typesof microchannel absorbers, microwick contactor absorbers and membranecontactor absorbers, are being developed. In absorption, the rate ofmass transfer is usually limited by the diffusion time of the gasspecies into the liquid. The characteristic diffusion time isproportional to the square of the film thickness, so the time to reach atarget gas saturation in the liquid is quartered every time the liquidfilm thickness is halved. Therefore, thin liquid channels defined bymicrowicks or membrane contactors provide improved mass transport andprocess intensification as compared to other absorption techniquesrelying on thicker liquid layers.

[0006] In wicks wetted by a sorbent, the liquid is preferably confinedto the wick volume, and therefore the liquid film thickness is relatedto the thickness of the wick. In a membrane contactor absorber, a liquidmicrochannel of defined thickness is separated from a gas microchannelby a porous membrane. In this case it is preferred that the sorbentliquid does not wet the contactor material so it does not weep orbreak-through the fine pores of the membrane into the gas channel. Ifthe pressure drop across the membrane is relatively high, as occurs whenthe liquid flow rate is increased sufficiently, break-through isobserved. It is difficult to recover from a membrane contactorbreak-through, because wettability and the pressure driving force do notfavor return of the liquid from the gas channel to the liquid channel.The analogous process upset in a microwick absorber is flooding, inwhich the feed flow rate of liquid exceeds the throughput of the wick.Since the microwick material is wetted, flooding will readily subsideinto the liquid flow structure when the feed rate is reduced. Achallenge for the microwick absorber technology is to create very thinliquid films (e.g., <100 μm) with high throughput. The present inventionhas overcome these challenges and provide a highly efficient method andapparatus for contacting a gas with a liquid in a microchannel.

[0007] In a first aspect, the invention provides a process of contactinga substance in a first fluid with a substance in a second fluid. In thisprocess, a gas comprising a first substance passes into a gas channel;and a liquid comprising a second substance passes into a fluidmicrochannel at a rate such that 1/Pe_(m) is about 30 or less. The fluidmicrochannel is adjacent to the gas channel. The dimensions of thisfluid microchannel is either defined by a wick (which is adjacent to thefluid microchannel), or a membrane that separates the fluid microchannelfrom the gas channel.

[0008] In a second aspect, the invention provides a microchannelcontactor, comprising: a gas channel; and a fluid microchannel that isadjacent to the gas channel and wherein the fluid microchannel isdefined by a wick, or that is separated from the gas channel by amembrane. The contactor is characterizable (i.e., capable of beingcharacterized) by a CO₂ loading capacity of at least 0.3 CO₂ mol per molDEA when a 2 M DEA aqueous solution is flowing through the fluidmicrochannel at a 2 M DEA flow rate set to establish 1/Pe_(m) to equal10, and when a 20% CO₂/80% N₂ mixture is passed into the gas channel ata level sufficient to preclude a gas phase mass transfer limitation.

[0009] U.S. patent application Ser. Nos. 09/588,871 filed Feb. 26, 2003,and 10/011,386 filed Dec. 5, 2001 are incorporated herein as ifreproduced in full below. These applications provide numerous methodsand apparatus for separating fluids and/or heat exchange. One processseparates fluids by passing a mixture of at least two fluids, comprisinga first fluid and a second fluid, into a device having at least onechannel. The channel has an open area and a wicking region. The firstfluid is either a liquid (such as a droplet or liquid particle) that issorbed by the wicking region, or a gas that, under separationconditions, forms a liquid in the wicking region. The first liquidtravels through the wicking region to a liquid flow channel and thenexits the device through a liquid exit channel. The second fluid is agas that passes through the gas flow channel to a gas exit, and exitsthe device through the gas exit.

[0010] Also disclosed is a process of contacting fluids in which atleast two fluids are passed into a device having at least one channel.The channel has an open area and a wicking region and an interfacebetween the wicking region and the open area. During operation, at leastone fluid flows through the wicking region, and at least one other fluidflows through the open area. At the interface between the wicking regionand the open area, one fluid contacts at least one other immisciblefluid, and there is mass transfer occuring through the interface betweenthe at least one fluid flowing through the wicking region, and the atleast one other fluid flowing through the open area.

[0011] The presence of wicks and optional pore throats and capturestructures are common to multiple embodiments of the invention. A wickis a material that will preferentially retain a wetting fluid bycapillary forces and through which there are multiple continuouschannels through which liquids may travel by capillary flow. Thechannels can be regularly or irregularly shaped. Liquid will migratethrough a dry wick, while liquid in a liquid-containing wick can betransported by applying a pressure differential, such as suction, to apart or parts of the wick. The capillary pore size in the wick can beselected based on the contact angle of the liquid and the intendedpressure gradient in the device, and the surface tension of the liquid.Preferably, the pressure at which gas will intrude into the wick shouldbe greater than the pressure differential across the wick duringoperation—this will exclude gas from the wick.

[0012] The liquid preferentially resides in the wick due to surfaceforces, i.e. wettability, and is held there by interfacial tension. Theliquid prefers the wick to the gas channel and as long as there iscapacity in the wick, liquid is removed from the gas stream and does notleave in the gas stream.

[0013] The wick can be made of different materials depending on theliquid that is intended to be transported through the wick. The wickcould be a uniform material, a mixture of materials, a compositematerial, or a gradient material. For example, the wick could be gradedby pore size or wettability to help drain liquid in a desired direction.Examples of wick materials suitable for use in the invention include:sintered metals, metal screens, metal foams, polymer fibers includingcellulosic fibers, or other wetting, porous materials. The capillarypore sizes in the wick materials are preferably in the range of 10 nm to1 mm, more preferably 100 nm to 0.1 mm, where these sizes are thelargest pore diameters in the cross-section of a wick observed byscanning electron microscopy (SEM). In a preferred embodiment, the wickis, or includes, a microchannel structure. Liquid in the microchannelsmigrates by capillary flow. The microchannels can be of any length,preferably the microchannels have a depth of 1 to 1000 micrometers (μm),more preferably 10 to 500 μm. Preferably the microchannels have a widthof 1 to 1000 μm, more preferably 10 to 100 μm. In a preferredembodiment, the microchannels are microgrooves, that is, having aconstant or decreasing width from the top to the bottom of the groove.In another embodiment, the microchannels form the mouth to a largerdiameter pore for liquid transport.

[0014] The wick is preferably not permitted to dry out during operationsince this could result in gas escaping through the wick. One approachfor avoiding dryout is to add a flow restrictor in capillary contactwith the wick structure, such as a porous structure with a smaller poresize than the wick structure and limiting the magnitude of the suctionpressure such that the non-wetting phase(s) cannot displace the wettingphase from the flow restrictor. This type of restrictor is also known asa pore throat. In preferred embodiments, a pore throat is providedbetween the wick and the liquid flow channel and/or at the liquidoutlet. In some embodiments, the wick can have a small pore diametersuch that is serves to transport fluids from the gas channel and alsoprevents gas intrusion, thus serving the dual purpose of a wick and apore throat.

[0015] A pore throat has a bubble point that is greater than the maximumpressure difference across the pore throat during operation. Thisprecludes intrusion of gas into the pore throat due to capillary forces(surface tension, wettability, and contact angle dependent). The porethroat should seal the liquid exit, so there should be a seal around thepore throat or the pore throat should cover the exit in order to preventgas from bypassing the pore throat. The pore throat is preferably verythin to maximize liquid flow through the pore throat at a give pressuredrop across the pore throat. Preferably, the pore throat has a pore sizethat is less than half that of the wick and a thickness of 50% or lessthan the wick's thickness; more preferably the pore throat has a poresize that is 20% or less that of the wick. Preferably, the pore throatis in capillary contact with the wicking material to prevent gas frombeing trapped between the wick and the pore throat and blocking theexit.

[0016] Flooding can result from exceeding the flow capacity of thedevice for wetting phase through the wick; the flow capacity isdetermined by the pore structure of the wick, the cross-sectional areafor flow, or the pressure drop in the wick in the direction of flow.

[0017] A capture structure can be inserted (at least partly) within thegas flow channel, and in liquid contact with the wick. The capturestructure assists in removing (capturing) a liquid from the gas stream.One example of a capture structure are cones that protrude from thewick; liquid can condense on the cones and migrate into the wick—anexample of this capture structure is shown in U.S. Pat. No. 3,289,752,incorporated herein by reference. Other capture structures includeinverted cones, a liquid-nonwetting porous structure having a pore sizegradient with pore sizes getting larger toward the wick, aliquid-wetting porous structure having a pore size gradient with poresizes getting smaller toward the wick and fibers such as found incommercial demisters or filter media. Mechanisms for capturing dispersedliquid particles include impingement (due to flow around obstructions),Brownian capture (long residence time in high surface area structure),gravity, centrifugal forces (high curvature in flow), or incorporatingfields, such as electrical or sonic fields, to induce aerosol particlemotion relative to the flow field.

[0018] Nonwetting surfaces can be disposed on the gas flow channelwalls. These nonwetting surfaces can help prevent formation of a liquidfilm on the surface and, in combination with a wick or a wick andcapture structure the liquid present in a fluid mixture can be siphonedaway from the condensing surface by capillary flow, thereby avoidingproblems associated with dropwise condensation, such as cold spots orre-entrainment.

[0019] The invention, in various aspects and embodiments can providenumerous advantages including: rapid mass transport, high rates of heattransfer, low cost, durability, and highly efficient liquid separationsin a compact space.

[0020] Devices and processes of the present invention are capable ofintegrating high efficiency, high power density heat exchange. Heatexchange can facilitate phase changes within the separation device, suchas condensation and evaporation. One example is partial condensation ofa gas stream to recover condensable components, such as water from thecathode waste gas stream from a fuel cell. Another optional feature isreduced or non-wettability of the wall adjacent to a heat exchangesurface to preclude formation of a liquid film. The heat transfercoefficient would increase substantially by avoiding the resistance of aliquid film.

[0021] The embodiments show preferred embodiments in which there aremultiple gas flow channels operating in parallel. This configurationallows high throughput and provides a large surface area to volume ratiofor high efficiency. In some preferred embodiments, layers are stackedto have between 2 and 600 separate gas flow channels, more preferablybetween 4 and 40 gas flow channels. As an alternative to the parallelarrangement, the channels could be connected in series to create alonger flow path.

[0022] Another advantageous feature of some preferred embodiments of theinvention is that the gas flow channels and/or liquid flow channels areessentially planar in the fluid separation regions. This configurationenables highly rapid and uniform rates of mass and heat transport. Insome preferred embodiments, the gas flow channels and/or liquid flowchannels have dimensions of width and length that are at least 10 timeslarger than the dimension of height (which is perpendicular to net gasflow).

[0023] The subject matter of the present invention is distinctly claimedin the concluding portion of this specification. However, both theorganization and method of operation, together with further advantagesand objects thereof, may further be understood by reference to thefollowing description taken in connection with accompanying drawingswherein like reference characters refer to like elements.

[0024] Glossary Of Terms

[0025] A “capture structure” is a structure disposed (at least partly)within a gas flow channel that assists movement of a liquid into thewick.

[0026] A “cell” refers to a separate component, or an area within anintegrated device, in which at least one unit operation is performed. Inpreferred embodiments, the cell has a width less than about 20 cm,length less than about 20 cm, and height less than about 3 cm.

[0027] “Device volume” refers to the entire volume of the device,including channels, headers, and shims.

[0028] “Entrainment” refers to transport of liquid into the gas exit

[0029] “Flow microchannel” refers to a microchannel through which afluid flows during normal operation of an apparatus.

[0030] A “laminated device” is a device having at least two nonidenticallayers, wherein these at least two nonidentical layers can perform aunit operation, such as heat transfer, condensation, etc., and whereeach of the two nonidentical layers are capable having a fluid flowthrough the layer. In the present invention, a laminated device is not abundle of fibers in a fluid medium.

[0031] A “liquid” is a substance that is in the liquid phase within thewick under the relevant operating conditions.

[0032] “Microchannel” refers to a channel having at least one dimensionof 5 mm or less. The length of a microchannel is defined as the furthestdirection a fluid could flow, during normal operation, before hitting awall. The width and depth are perpendicular to length, and to eachother, and, in the illustrated embodiments, width is measured in theplane of a shim or layer.

[0033] “Microcomponent” is a component that, during operation, is partof a unit process operation and has a dimension that is 1 mm or less.

[0034] “Microcomponent cell” is a cell within a device wherein the cellcontains microcomponents.

[0035] “Pore throat” refers to a porous structure having a maximum poredimension such that a non-wetting fluid is restricted from displacing awetting fluid contained with the pore throat under normal operatingconditions.

[0036] “Residence time” refers to the time that a fluid occupies a givenworking volume.

[0037] A “substance” is a chemical compound or molecule.

[0038] “Unit process operation” refers to an operation in which thechemical or physical properties of a fluid stream are modified. Unitprocess operations (also called unit operations) may includemodifications in a fluid stream's temperature, pressure or composition.

[0039] A “wicking region” is the volume occupied by a wick, or, awicking surface such as a grooved microchannel surface.

[0040] “Working volume” refers to the total channel volume of thedevice, and excludes the headers and solid shim and end plate materials.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 is a cross-sectional view of a gas/liquid separator.

[0042]FIG. 2 is a top-down view of a gas flow channel layer of thegas/liquid separator.

[0043]FIG. 3 is a bottom-up view of a liquid flow channel layer of thegas/liquid separator.

[0044]FIG. 4 is a top-down view of an end plate of the gas/liquidseparator.

[0045]FIG. 5 illustrates the other end plate of the gas/liquidseparator.

[0046]FIG. 6 is a cross-sectional view of a gas/liquid contactor.

[0047]FIG. 7 is a top-down view of a gas flow channel layer of thegas/liquid contactor.

[0048]FIG. 8 is a bottom-up view of a liquid flow channel layer of thegas/liquid contactor.

[0049]FIG. 9 illustrates a wick insert.

[0050]FIG. 10 is a cross-sectional view of a counter-current heatexchange condenser assembly.

[0051]FIG. 11 is a top-down view of a gas flow channel layer of thecounter-current heat exchange condensor assembly.

[0052]FIG. 12 is a bottom-up view of a liquid flow channel layer of thecounter-current heat exchange condensor assembly.

[0053]FIG. 13 is a top-down view of a heat exchange layer of thecounter-current heat exchange condenser assembly.

[0054]FIG. 14 is a cross-sectional view of a cross-current heat exchangecondenser assembly.

[0055]FIG. 15 is a top-down view of a gas flow channel layer of thecross-current heat exchange condensor assembly.

[0056]FIG. 16 is a bottom-up view of a liquid flow channel layer of thecross-current heat exchange condensor assembly.

[0057]FIG. 17 is a top-down view of a heat exchange layer of thecross-current heat exchange condenser assembly.

[0058]FIG. 18 is a data plot from the Examples showing maximum waterflow rate with no entrainment as a function of gas flow rate.

[0059]FIG. 19 is a plot of carbon dioxide loading in 2M DEA/water as afunction of the inverse Peclet number for varying liquid film thicknessin three microwick contactors and one membrane contactor.

[0060]FIG. 20 is a plot of carbon dioxide flux into a 2M DEA/PEGsolution in a microchannel membrane contactor at three liquid flowrates.

DETAILED DESCRIPTION

[0061] In a first aspect, the invention provides a gas/liquid separator.An embodiment of on such device is illustrated in FIG. 1. Theillustrated device is made up of end plates 6, 8 and alternating centralshims 1 and 2. A fluid inlet 9 is connected to open channel 12. Shim 1has open gas flow channels 14. The surface 18 of solid section 16 formsthe bottom of the gas flow channel. The top of the gas flow channel isformed wick 22 of shim 2. As the gas/liquid mixture flows throughchannel 14, the liquid component is absorbed by the wick 22. The liquidin the wick travels to a wick exit channel and flows out through liquidoutlet 29. To remove liquid suction can be applied through a pump (notshown). Gas flows out through a separate channel and out through gasoutlet 19.

[0062] A top down view of shim 1 is illustrated in FIG. 2. Channels 14are separated by lands 32. The lands can support a wick insert andprevent channel collapse during fabrication. Gas flows through channels14 and into gas exit holes 34. Lands 32 are preferably the same heightas edges 36. The height of the gas flow channels 14, from surface 18 towick surface 24 is preferably about 10 μm to 5 mm, more preferably 100μm to 1 mm. The height of the channels is preferably small for good heatand mass transfer and overall device size, balanced against potentiallyslower flow rates. The path to exit wicks 42 can be blocked by lands 38.A high ratio of surface area of exposed wick to volume of gas flowchannel is desirable for efficient phase separations. Preferably thisratio is from 1 to 1000 cm²:cm³, more preferably from 5 to 100.

[0063] A bottom up view of shim 2 (without wick) is illustrated in FIG.3. It includes gas exit holes 34 (open space) and exit wicks 42. A wick(not shown) may be inserted open space 42 (preferably without blockinghole 12). Alternatively, surface of the shim can be a wick structuresuch as microchannels. In any event, the wick structure should create acontinuous liquid flow path with exit wick 42, but should not block gasexit holes 34.

[0064]FIG. 4 illustrates a top down view of end plate 6 showing spacesfor gas/liquid entry 12 and gas exit holes 34. FIG. 5 illustrates endplate 8 with exit wicks 42. Of course, the device could be plumbed tohave gas and liquid exit from the same side or the gas/liq entry couldbe moved to the opposite end plate 8 to have liquid exit and gas/liqentry on the same side of the device.

[0065] In operation of a device with a wick, the wick should not beflooded, and it is preferably not dry. A wet or saturated wick willeffectively transport liquid through capillary to a low pressure zone,such as low pressure created by suction. A pore throat may be added toliquid outlet 27 to prevent gas flow out of liquid exit.

[0066] A cross-sectional view of a gas/liquid contactor is illustratedin FIG. 6. This contactor has end plates 54 and 56 and alternating shims51 and 52. T-joint inlets 60 and 62 are for the passage of gas andliquid respectively. T-joint outlets 64 and 66 are for the passage ofgas and liquid respectively. A top-down view of shim 51 is illustratedin FIG. 7. Gas flows in through gas inlet holes 76 and out throughoutlet holes 72. A bottom-up view of shim 52 is illustrated in FIG. 8.Liquid flows in through wick channels 82 through the wick and outthrough wick channels 84. A wick insert is illustrated in FIG. 9. Thewick insert has through holes 92 for gas flow through the wick. Regions94 of the insert can be continuous wick or can be holes that are filledwith a continuous wick through the wick channels. Where the wick insertcovers regions 94, disks or other inserts of a wicking material shouldbe disposed in channels 78, 79, 82 and 84 to provide a continuouscapillary liquid flow path. Use of microchannels in shim 52 can obviatethe need for a wick insert. Endplate 54 has wick channels (not shown)corresponding to the wick channels in shim 52. Endplate 56 has gas inletand outlet holes (not shown) corresponding to inlet and outlet holes 72and 76. This device illustrates a preferred counterflow of liquid andgas phases. Where the liquid is used to selectively absorb componentsfrom the gas phase, the counterflow construction contacts the gas withthe lowest concentration of extractable components with the purestliquid and thus provides for maximal absorbtion of the gas components.

[0067] A cross-sectional view of a counter flow heat exchange condenserassembly 100 is illustrated in FIG. 10. The assembly includes shims 101,102, and 103, inlets 104 and 105, and outlets 106, 108, and 110. Shim102 includes wick 112 and wall 124. Shim 101 has gas flow channels 114and walls 126. Heat exchanger shim 103 contains microchannels 166 andwall 122.

[0068] A top down view of shim 101 is illustrated in FIG. 11. Theillustrated shim contains gas flow channels 118, fluid flow inlets 113,lands 116, gas exit holes 117, heat exchange fluid holes 115, and liquidexit hole 119.

[0069]FIG. 12 illustrates a bottom up view of shim 102. The illustratedshim includes liquid flow channel 128, fluid flow inlets 123, gas exitholes 127, heat exchange fluid holes 125, and liquid exit hole 129.

[0070] A top down view of a heat exchange shim is illustrated in FIG.13. The heat exchange fluid enters through holes 132, travels throughmicrochannels 134 and exits through holes 136. Passageways 137, 138, and139 are provided for fluid mixture, gas and liquid to flow through theheat exchange shim.

[0071] Endplate 190 has inlet and outlet holes for the heat exchangefluids. End plate 111 has fluid inlet holes, gas outlet holes, andliquid exit holes.

[0072] During operation, a fluid mixture can enter through inlet 105 andpass through gas flow channels 114. A heat exchange fluid enters throughinlet 104 and passes through microchannel layer 116. In a preferredembodiment, one component of the fluid mixture mixture condenses in wick112. Heat can be removed from (or added to) the system by the heatexchange fluid. Depending on system requirements, either wall 101 orwalls 122, 124 can be insulating. In a preferred embodiment, the layers(shims) are arranged to have the repeating sequence: wick, gas flowchannel, wall, microchannel layer, wall, gas flow channel, and wick.

[0073] A cross-current heat exchange assembly is illustrated in FIGS.14-17. The gas and liquid inlets and outlets and the lands are indicatedusing the same shadings as above. In this embodiment, the heat exchangefluid runs cross-current (at a right angle) to the flow of fluid throughthe gas flow channel.

[0074] The figures illustrate preferred embodiments in which liquidflows by direct contact of the wick in the liquid flow channels to awick in the liquid exit channels. In other embodiments, however, theliquid, after having been sorbed into the wick, can flow into a liquidflow channel that does not contain a wick. The liquid can then flow outthrough wicks or by fluid flow without wicks.

[0075] The shims can be made of metals, plastics, ceramic or compositematerials. Metal shims can be made by etching, conventional cutting andmachining, electrical discharge machining (EDM), laser machining,stamping, or molding techniques. Plastic shims can be made using thesame techniques or by conventional plastic forming techniques, includinginjection molding, hot embossing, stamping, casting, and other moldingtechniques. Ceramic shims could be made using techniques well known forfabricating ceramic parts, including those used in fabricating solidoxide fuel cell elements. The shim material facing the gas channel canbe made hydrophobic through coatings, treatment or by the choice ofmaterial. The end plates are preferably made of a similar material asthe shims and made using similar techniques. The shims are stacked withwicks installed within the liquid flow channel, either held in looselyby the lands and channel walls or by adhering the wicks to the liquidchannel wall. The wick or pore throat must prevent an open path for thegas to flow to the liquid exit. This is accomplished either by closetolerances between the wick or pore throat structure and the walls, byusing a sealant, such as epoxy to install the wick or pore throat, orthrough the use of gaskets or o-rings. End plates are then placed on thetop and bottom of the shim stack. The seams between shims and betweenend plates and shims are either sealed by bonding or by a compressionseal. Bonding can be accomplished by diffusion bonding, by chemicalreaction, such as using an epoxy resin, or by gluing with an adhesivematerial. A compression seal can be accomplished using gaskets, O-rings,or by surface to surface contact and bolting the device together. Inletsand outlets can be connected by the same methods, by welding, by screwsor bolts, or by other known connection techniques.

[0076] When making low volumes of a given device or when the deviceneeds to be disassembled for cleaning or modifications, the preferredfabrication technique is to fabricate the shims and end plates byconventional machining, such as by milling. Seals between the shims andend plates are preferably accomplished by compression seals using eitherO-rings placed in grooves machined into one of the sealing surfaces orby using a gasket material cut to match the profile of the sealingsurface. The device is then held together in compression using bolts.The liquid exit is isolated from the gas flow channel using gaskets orsealant, such as epoxy, between the wick or pore throat and the wall.Inlets and outlets can be installed using standard threaded fittings orother known connection techniques.

[0077] The devices and processes described herein are especiallydesirable for integration in a system. These systems can accomplishdesired functions such as heat transfer, mass transfer, heterogeneousreaction, electrochemical reactions, or electric field enhancements.When integrated as an element in a chemical reactor, the inventivedevices can result in process intensification (e.g., reduced masstransport residence times) and/or greater than equilbrium conversion andselectivity for chemical reactions. Combination with a heat exchangercan facilitate multi-phase endothermic or exothermic chemical reactions.

[0078] Heterogeneous catalytic reactions can also be accomplished withinthe proposed architecture by impregnating active catalyst materials inthe wicking structure for liquid phase reactions and/or in the capturestructure for gas phase reaction. One example is Fischer-Tropschsynthesis, where condensable hydrocarbons are produced. Reactorresidence time can be reduced by the incorporation of structures thatremove liquid hydrocarbon products in contact with the catalyststructure. The devices can also be used in an integral reactor-chemicalseparator. For example, the wick or an absorbent material within thewick can selectively remove one of the products. This causes a shift inthe equilbrium conversion as well as improved selectivity. Other,nonlimiting, examples include low temperature water gas shift reaction,where we believe that selectively removing CO could lower the operatingtemperature for achieving adequate conversion. This effect could beenhanced by flowing an absorbent liquid countercurrent to a flowing gasstream.

EXAMPLES Example 1

[0079] Half inch polycarbonate was machined to create a 3000 μm deep×2cm wide×8 cm long channel. An ⅛″ NPT barbed fitting placed 1 cm from theend of the channel served as the liquid outlet. A 2 cm×8 cm piece of70×70 stainless steel mesh available from McMaster Carr was placed inthe channel beneath a {fraction (1/16)}″×2 cm×8 cm piece of sinterednickel with 5 □m pores (Mott Corporation). The sintered nickel wassealed into place using Loctite® RTV silicone adhesive.

[0080] A 0.25″ piece of polycarbonate was machined so that two ⅛″ NPTbarbed fittings could be threaded into holes 6 cm apart. This piece ofpolycarbonate had been made hydrophobic by treatment in a capacitivelycoupled RF (13.56 MHz) plasma reactor (66×66×91 cm³) using two stainlesssteel parallel electrodes (25 cm in diam.). The electrodes wereseparated by a distance of 10 cm, and were both water-cooled. The RFpower was applied to the upper electrode, and the sample was placed onthe lower, grounded electrode. An automatic L-type matching network withtwo air variable capacitors converted the complex impedance of plasmasto 50 resistivity. A dc self-bias meter with selectable scaling allowedprecise control over the matching/chamber environment. The system wasfirst evacuated to a base pressure of 10⁻⁵ Torr using a diffusion pump.The CF₄ gas was then introduced to the system and a pressure of 100mTorr was established by adjusting the opening of a throttle valveplaced between the diffusion pump and the chamber. After a stabilizationperiod of several minutes, the plasma was initiated. The treatment wasperformed at a power of 100W, a pressure of 100 mTorr, and a gas flowrate of 50 sccm for 5 minutes. (See “In-situ and real-time monitoring ofplasma-induced etching of PET and acrylic films”, M. K. Shi, G. L.Graff, M. E. Gross, and P. M. Martin, Plasmas and Polymers, in press).The advancing contact angle of water on the plasma treated substrate wasgreater than 110°.

[0081] The half inch and quarter inch polycarbonate pieces were boltedtogether and sealed with an o-ring such that the 3000 μm channel andhydrophobic surface faced each other. The device volume was 90 cm³. Thedevice was situated so that the liquid outlet faced downwards. A 3″piece of ⅛″ tubing was attached to the liquid outlet to provide suctionby siphoning. The fitting in the ½″ polycarbonate piece above the liquidoutlet served as the gas outlet, while the other fitting in the ½″polycarbonate served as the inlet to a mixture of air and water. Variousflows of air were fed to the device. The water flow rate was adjusted tothe maximum flow that allowed no entrainment of water in the gas exitline. This maximum flow is plotted as a function of gas flow rate(indicated as Mott in FIG. 18). As can be seen, the maximum water flowrate was constant at roughly 6.5 milliters/min (mL/min) between the airflow rates of 500 and 1250 mL/min. Significant entrainment of the liquidoccurred at 1500 mL/min air flow rate, until the liquid flow rate wasdecreased to 2.6 mL/min. No gas was observed to exit the liquid channel.Given that the working volume of the channels is 4.8 cm³, the residencetime is 0.2 seconds at the highest flow rate.

Example 2

[0082] The device was the same as above, except a 1400 μm channelreplaced the 3000 μm channel. This channel had a 8 cm×2 cm piece of0.0011″ Supramesh from Pall Corporation placed in the bottom. Thismaterial consists of sintered stainless steel overlying a fine stainlessmesh. Flat gasket material with a 1.5 cm×7.5 cm hole placed inside thechannel served to seal both pieces of polycarbonate together as well aspreventing gas intrusion into the porous material.

[0083] Again, the gas flow rate was plotted against the maximum liquidflow rate that allowed no entrainment of water in the gas exit line.These data are indicated as Pall in FIG. 18. As can be seen, the waterflow rate was close to 10 mL/min between the air flow rates of 200 and1500 mL/min. At a gas flow rate of 1750 mL/min, entrainment occurred inthe air exit until the liquid flow was decreased to 5 mL/min.

[0084] A comparison of the efficiency of this device with a typicalcentrifugal separator demonstrates the economy of size and weight ofthis invention. A known fuel processing stream contains 355 L/min of gaswith 300 mL/min water. The size of a type T cast iron gas/liquidseparator from Wright-Austin to treat this stream would be approximately2100 cm³ and weigh 14 pounds. The residence time within the device is0.35 seconds. In contrast, the invention would have a working volume ofapproximately 450 cm³ giving a residence time of 0.08 seconds. Thepressure drop across the Wright-Austin system is estimated to be 1.38inches of water, while the pressure drop across the invention isestimated to be 0.075 inches of water.

Example 3

[0085] The effectiveness of membrane and wick microchannel contactorshas been demonstrated experimentally. CO₂ was absorbed from a20%-CO₂:80%-N₂ mixture into a 2-M diethanolamine/water (DEA) solution.For these wick tests, a single-gas-channel, plate-type contactor wasused (similar to that shown in FIG. 1 but with only 1 gas channel andone wick layer). The membrane contactor absorber was similarlyconfigured except for: the use of a membrane contactor instead of amicrowick; heat exchange channels sandwiching the gas channel and liquidchannel; and different microchannel dimensions. In each device, theCO₂-laden gas was delivered via a gas inlet counter-current to theliquid stream. Gas flow was maintained at a sufficient level to precludegas phase mass-transfer limitations. The composition of the effluent gasstream was determined with a residual gas analyzer, and the change incomposition was used to evaluate CO₂ mass transfer into the flowing DEAsolution.

[0086] Results for three microwick contactor experiments using wicks ofvarious materials and thicknesses (150 μm, 250 μm, and 380 μm) and amembrane contactor test using a 100-μm liquid microchannel are presentedin FIG. 19. The results of all the experiments are correlated in termsof the inverse mass-transfer Peclet number (Pe_(m) ⁻¹), which is definedas the solute diffusivity (DEA diffusion, in this case) multiplied bythe liquid residence time divided by the thickness of the liquid channel(which, in the case of the wick, is the wick channel) squared.Equilibrium uptake of CO₂ is sensitive to temperature and CO₂ partialpressure and is 0.5-0.65 moles CO₂ per mole of DEA at the conditionstested (room temperature, 20-25° C.). The objective was to achieve highsolvent loading with a minimum residence time, as this would lead toreduced sorbent mass in a separation/purification system. Nearequilibrium loading of CO₂ is achieved at Pe_(m) ⁻¹ above 10, whereperformance is limited by solvent capacity for the solute. Thus, it wassurprisingly discovered that superior results could be obtained bycontrolling residence time of the liquid as a function of liquid layerthickness so that 1/Pe_(m) is short. 1/Pe_(m) should be about 30 orless, preferably about 20 or less, and more preferably about 10 or less,and in some embodiments about 5 to about 15.

[0087] Overall mass-transfer coefficients were also determined from thedata shown in FIG. 19 and were compared to those typical of conventionalpacked towers using 2-M monoethanolamine/water solutions (Kohl, A. L.and R. B. Nielsen, Gas Purification, 4^(th) Ed., Gulf PublishingCompany, Houston, Tex., 1985). The overall mass transfer coefficientswere as much as 2.6 times greater for the thinnest microwick and about7.1 times greater for the membrane system.

[0088] For space applications, water is not likely the preferred solventfor absorption systems because of its relatively high vapor pressure.Using the membrane contactor absorber described above, we completed aseries of experiments in which the solvent water was replaced with muchless volatile polyethylene glycol (PEG). The microchannel absorberincorporated heat exchangers on both the liquid and gas sides, and thisfeature was used in several experiments to evaluate mass transfer ratesas a function of temperature in the range ˜25 to ˜80 C. Results areshown in FIG. 20. Reduced liquid viscosity with increasing temperaturehad a two-fold benefit: first, the mass transfer rate increased, mostlikely due to increased mass diffusivity in the liquid; and second, theliquid pressure drop across the device was reduced. This allowed higherliquid flow rates to be evaluated without liquid breakthrough across thecontactor membrane. At a given sorbent flow rate (residence time),increasing the temperature of absorption above ambient also resulted inan increased conversion of DEA to amine-CO₂ product (FIG. 20). Thus, itwas unexpectedly discovered that improved results could be obtained byincreasing temperature above ambient. For example, improved CO₂scrubbing efficiency was observed at a temperature in the range of about40 to about 60° C. The decrease in CO₂ sorption at higher temperaturesis expected, as the reverse reaction leading to desorption (or lessabsorption) of CO₂ from the amine is favored. This is the basis of acontinuously regenerated thermal swing absorption process. Also note inFIG. 20 that the CO₂ flux increases as the absorbent residence timedecreases. The flux is higher when the CO₂ loading of the sorbent is lowand Pe_(m) ⁻¹ is small (FIG. 19).

[0089] Because of the relatively high viscosity of PEG and acorresponding reduction in DEA diffusivity compared to that in water,mass transfer rates were lower for DEA/PEG. At room temperature, the CO₂mass flux to a 2-M DEA/water solution was 5 to 10 times greater thanthat for comparably configured devices using a 2-M DEA/PEG sorbent.Since water may not be an acceptable solvent for space applications,this is further indication of the need for microchannel liquid absorbers(microwick or membrane contactor) integrated with heat exchange.Advancements in wicking microtechnology will result from thinner, higherpermeability, and lower pore size wicks. Membrane contactor absorberswill benefit from robust membranes that are less susceptible tobreak-through. Both microwick and membrane contactor absorptionapproaches are amenable to scale up in multi-channel systems.

[0090] Closure

[0091] While preferred embodiments of the present invention have beenshown and described, it will be apparent to those skilled in the artthat many changes and modifications may be made without departing fromthe invention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

1. A process of contacting a substance in a first fluid with a compoundin a second fluid, comprising: passing a gas comprising a firstsubstance into a gas channel; passing a liquid comprising a secondsubstance into a fluid microchannel at a rate such that 1/Pe_(m) isabout 30 or less; wherein the fluid microchannel is adjacent to the gaschannel and wherein the fluid microchannel is defined by a wick, orwherein a membrane separates the fluid microchannel from the gaschannel.
 2. The process of claim 1 wherein the liquid in the fluidmicrochannel is heated to a temperature above room temperature.
 3. Theprocess of claim 1 wherein the fluid microchannel is defined by a wickand comprising passing the liquid comprising a second substance into afluid microchannel at a rate such that 1/Pe_(m) is about 20 or less. 4.The process of claim 2 wherein the first substance is carbon dioxide. 5.The process of claim 3 wherein the first substance is carbon monoxide.6. The process of claim 3 wherein the wick has a thickness of 380microns or less.
 7. The process of claim 3 wherein the wick has athickness of 150 to 380 microns.
 8. The process of claim 4 wherein thefluid microchannel is heated to a temperature of about 40 to about 60°C.
 9. The process of claim 1 comprising passing the liquid comprising asecond substance into a fluid microchannel at a rate such that 1/Pe_(m)is about 5 to about
 20. 10. The process of claim 3 wherein the wick hasa width, length and height that are mutually perpendicular, wherein theheight is perpendicular to an interface between the liquid and the gas,and wherein the height of the wick is at least five times smaller thanboth the width and the length of the wick.
 11. The process of claim 8wherein the equilibrium sorption of carbon dioxide in the liquid ishigher at room temperature that it is at 50° C.
 12. A microchannelcontactor, comprising: a gas channel; a fluid microchannel that isadjacent to the gas channel and wherein the fluid microchannel isdefined by a wick, or that is separated from the gas channel by amembrane; wherein the contactor is characterizable by a CO₂ loadingcapacity of at least 0.3 CO₂ mol per mol DEA when a 2 M DEA aqueoussolution is flowing through the fluid microchannel at a 2 M DEA flowrate set to establish 1/Pe_(m) to equal 10, and when a 20% CO₂/80% N₂mixture is flowing into the gas channel at a level sufficient topreclude a gas phase mass transfer limitation.
 13. The contactor ofclaim 12 wherein the fluid microchannel is defined by a wick that has awidth, length and height that are mutually perpendicular, wherein theheight is perpendicular to an interface between the fluid microchanneland the gas channel, and wherein the height of the wick is at least fivetimes smaller than both the width and the length of the wick.
 14. Thecontactor of claim 12 wherein the gas channel comprises carbon dioxide.15. The contactor of claim 12 wherein the gas channel is a microchannel.16. The contactor of claim 12 further comprising a microchannel heatexchanger that is in thermal contact with the gas channel.
 17. Thecontactor of claim 12 further comprising a microchannel heat exchangerthat is in thermal contact with the gas channel.